1. Field of the Invention
This invention relates to musical tone synthesis and in particular is concerned with an improvement for producing tones from stored incremental harmonic values.
2. Description of the Prior Art
The most obvious method to imitate an acoustic musical instrument is to record the sound and to replay the recording in response to an actuated keyswitch on an array of keyswitches. While at first thought the straightforward technique of recording and keyed playback seems to be attractive, a practical realization of such a musical instrument can be burdened by a large amount of memory required to store the recorded data. The maximum amount of memory is associated with a tone generation system that uses a separate and distinct recording for each note played in the range of the musical instrument's keyboard. Some economy in the memory requirement has been made by using a single recording for several contiguous musical notes. This economy is based upon the tacit assumption that the waveshape for the imitated musical instrument does not change markedly between several contiguous successive notes.
Electronic musical tone generators that operate by playing back recorded musical waveshapes stored in a binary digital data format have been given the generic name of PCM (Pulse Code Modulation). A musical instrument of the PCM type is described in U.S. Pat. No. 4,383,462 entitled "Electronic Musical Instrument." In the system described in the patent, the complete waveshape of a musical tone is stored for the attack and decay portions of the musical tone. A second memory is used to store the remainder of the tone which comprises the release phase of the musical tone. The sustain phase of the musical tone is obtained by using a third memory which stores only points for a single period of a waveshape. After the end of the decay phase, the data stored in the third memory is read out repetitively and the output data is multiplied by an envelope function generator to create the amplitude variation for the sustain and release portions of the generated musical tone.
Various techniques have been employed to reduce the amount of stored data in a PCM tone generation system. A method of reducing the amount of stored data is described in U.S. Pat. No. 3,763,364 entitled "Apparatus For Storing And Read Out Periodic Waveforms." The disclosed system utilizes the even and/or odd symmetry of a waveform in such a manner that only sample points for one-half of the waveform need be stored in a waveshape memory. When the waveform is read from the waveshape memory, the unstored sample points of the second half of the waveshape period are recreated by using the symmetry property of the stored data points.
A method for reducing the amount of stored data in a PCM musical tone generation system is described in the copending Patent Application No. 827,983 filed Feb. 10, 1986 entitled "Data Reduction For A Musical Instrument Using Stored Waveforms." This referenced application has the same inventor as the present application and they are assigned to the same assignee. In the system described in the referenced copending application a waveshape memory is used to store a number of segments of waveshape data points. Each segment corresponds to one-half of the period of a synthesized waveshape having a waveshape symmetry about the half-wave point and having a spectra equal to that of a corresponding segment of a musical tone recorded from a musical instrument. The missing waveshape points are reconstructed by reading out each segment in a forward and reverse memory order and then jumping to an adjacent segment where the forward and back operation is repeated. A further reduction in the amount of stored data can be obtained by repeating the data read out for a given segment of waveshape memory a predetermined number of cycles before a jump is made to an adjacent segment.
A penalty must usually be paid for a scheme that reduces the number of stored data points in a PCM musical tone generation system. A reduction in the amount of stored data generally is accompanied by an increase in the complexity of the tone generation system. Even with the current low cost of microelectronic memory devices, the large number of stored data points in a simple PCM tone generation system is too large for implementing a low cost keyboard musical instrument. For this reason it is attractive to employ some data processing subsystem logic as a means for reducing the amount of waveshape memory. An object of the present invention is to implement a musical tone generation having the tone capability of a PCM system but system but using only a minimal stored set of data for each of the selectable musical tone colors.